Mozilla has shipped a new pre-release build of Firefox 4 with its latest JavaScript technology, JaegerMonkey, baked in. To try it out, you’ll need to install one of Mozilla’s nightly JS Engine Preview builds. The JaegerMonkey-equipped browser was made available Thursday.

Mozilla has shipped a new pre-release build of Firefox 4 with its latest JavaScript technology, JaegerMonkey, baked in.

To try it out, you’ll need to install one of Mozilla’s nightly JS Engine Preview builds. The JaegerMonkey-equipped browser was made available Thursday. Keep in mind, this is pre-release software and should be used only for testing.

JaegerMonkey has a dramatic impact (in the positive direction) on Firefox’s core JavaScript engine, TraceMonkey, which shipped with version 3.5 of the browser.

It’s a welcome addition. Chrome, Safari and Opera have all made significant enhancements to their JavaScript capabilities since Firefox 3.5′s release. Internet Explorer 9, due to arrive as a beta this month, is on par with those browsers when it comes to script execution. If all goes well, this new code will be included in Firefox 4 when it ships later this year.

In my informal testing, the new build is much, much faster at running script-heavy websites and demos than the current Firefox 4 beta. It’s about as fast as Chrome on my Mac, which says a lot, considering how much of a speed demon Google’s browser is. All of those JavaScript demos and games that were only really smooth enough to impress in Chrome and Safari are now nice and fluid in Firefox.

You can see some of Mozilla’s recent test data on JavaScript benchmarks (here and here) to get a clearer picture of how JaegerMonkey is helping close the gap to the other browsers.